2019-06-01_Motorcycle_Mojo_Magazine

MAY 2019 MOTORCYCLE MOJO 63

Technical articles are written purely as

reference only and your motorcycle may

require different procedures. You should

be mechanically inclined to carry out your

own maintenance and we recommend you

contact your mechanic prior to performing

any type of work on your bike.

three. On a bank of four carburetors,

each outer screw adjusts the throttle

valves of the adjoining outer carbure-

tors, and the centre screw adjusts the

throttles of the left- and right-paired

carburetors to each other.

To set the throttle valves equally,

you can use a solid wire or a very small

drill bit as a gauge to measure the gap

between the throttle valve and the

carburetor bore. The smaller the wire

or drill bit, the more accurately you can

adjust the throttle valves, since at small

throttle openings it requires a very

small adjustment at the screws to see a

measureable difference, as opposed to

when the throttles are opened wider. A

wire or drill bit that measures less than

1 mm is ideal; I used a number 65 bit to

adjust the FZ750 carbs pictured here.

On a four-cylinder engine, the carbu-

retors can be adjusted in pairs. The best

way to hold the throttle steady while

synchronizing is to turn in the idle

speed screw until you feel a slight drag

on your measuring gauge at the throttle

valve of the “base” carburetor. The

base carburetor is the one connected

directly to the throttle cable linkage,

and on a four-cylinder engine, that’s

usually cylinder number two. Note the

number of turns of the idle screw from

the starting point so that you can put it

back after you’re done. You can further

readjust the idle to spec once the engine

is warmed after reassembly.

Begin by setting the gap at the base

carb, which won’t change when the

other carburetors are adjusted. Next,

adjust the throttle gap at its adjoining

carb (number 1) accordingly, using the

adjusting screw. Move to carburetors

3 and 4, setting their throttle valves

evenly, without, at this point, taking

into account the adjustment of carbure

• 
  

tors 2 and 3. Once you’ve adjusted carbs

3 and 4 evenly, you can use the centre

adjusting screw to balance out the two

outside pairs of carburetors. All you

need to do at this point is to check the

gap at carbs 2 and 3, since carbs 1 and 4

will follow their mating carburetors.

With the carbs off the bike, you can sync the carbs

when using a very small drill bit as a feeler gauge

ensuring each carb butterÀy is adjusted the same.

In addition to the liquid-¿lled vacuum

gauge, there is also dial-type vacuum

gauges. Whichever you use, the end

result should be the same.

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Done carefully, the carburetors will

need no further synchronization once

back on the bike. However, this does

not mean you’ll get even vacuum at the

intake manifolds should you verify it.

If the engine is in excellent condition,

the vacuum readings should be spot on.

However, if the vacuum readings are

uneven, this indicates that the cylinders

with low intake vacuum are low on

compression, which means there’s likely

more work to do down the road.

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